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* Replace deprecation warning f-string with class name.

When `__repr__` is invoked in the instance serialization of
`config_dict` fails, because it contains `kwargs` of type `<class
inspect._empty>`.

* Revert "Replace deprecation warning f-string with class name."

This reverts commit 1c4eb8cb104374bd84e43865fc3865862473799c.

* Do not attempt to register `"kwargs"` as an attribute.

Otherwise serialization could fail.
This may happen for other attributes, so we should create a better
solution.